As an acidic waste liquid containing highly concentrated copper ions (hereinafter referred to as copper-containing acidic waste liquid), there have been known an etching waste liquid resulting from a process of etching a copper printed board in an etching solution of cupric chloride, a waste liquid after renewing a plating bath liquid in a process of producing electrolytic copper foil, and the like. These waste liquids contain copper whose concentration is as high as 5-20 percent by mass (hereinafter simply referred to as %), and contain coexisting chloride ions and sulfate ions whose concentrations are normally as high as 5-30%.
As a recovery treatment of copper from the copper-containing acidic waste liquid, there has been partly conducted a method of recovering metal copper precipitated by a reaction between copper ions and scrap iron caused by differences of ionization tendency of copper and iron. However, according to this method, a recovery rate of copper from the waste liquid is low, and there still remains waste liquid which contains iron ions eluted by the reaction with copper ions, and uncollected copper ions. Thus, it is hard to say that this method is an efficient treatment method because treatment of the remaining waste liquid is separately required.
Further, as a commonly-used method, there has been known a treatment method of removing heavy metals in the form of hydroxide precipitated by adding alkaline substances such as sodium hydroxide to the waste liquid. However, this method is unsuitable for treating the copper-containing acidic waste liquid which has a high concentration of copper ions because resultant sludge is bulky and large in amount.
Furthermore, as a treatment of the etching waste liquid, there has been attempted a treatment method for recovering copper in the form of copper oxide by adding alkaline substances in the waste liquid to change copper ions into insoluble hydroxide followed by adding an oxidizing agent to produce copper oxide (see e.g. Japanese laid-open patent publication No. 2004-50096 (patent application No. 2002-212857)). However, in this technique, when hypochlorite, chloride of lime or the like, each of which contains chloride ions, is used as the oxidizing agent, the concentration of chloride ions in the waste liquid after addition of the oxidizing agent becomes higher, thereby causing problems such as a formation of double salt of copper chloride and copper oxide and undesirable incorporation of salt content into the sludge. If the waste liquid is a liquid having a low concentration of copper ions, less than a few percent, such as washing wastewater after an etching process, this technique does not really cause a problem. However, in the case where the waste liquid that contains a high concentration of copper ions is treated, there remain problems to be solved, such as a problem of an increase of impurities contained in the recovered copper oxide.
On the other hand, when hydrogen peroxide is used as an oxidizing agent, the above-mentioned increase of salts does not occur. However, an efficient treatment of the waste liquid cannot be carried out because of the following problems. Specifically, when treating a strong acidic waste liquid having copper ions and chloride ions or sulfate ions which coexist in high concentrations, a solid material whose main component is a double salt of copper hydroxide and copper chloride or a double salt of copper hydroxide and copper sulfate is precipitated under conditions of a pH of about 1.5 or higher in a process of neutralization by adding an alkaline agent to the strong acidic waste liquid to change the pH level from an acidic property to a neutral or alkaline property. The solid material whose main component is the above double salt has a high concentration of impurities and is bulkier than copper oxide, and hence the recovery treatment efficiency is low. In particular, when treating the waste liquid containing highly-concentrated copper, the waste liquid changes into pasty sludge in the course of neutralization, thus causing difficulty in its treatment.
Further, the above double salts are hard to be decomposed by oxidization with hydrogen peroxide, and the double salts act as catalyst for decomposition of hydrogen peroxide. Therefore, even if hydrogen peroxide is added as an oxidization agent to the liquid in which the solid material has been precipitated, hydrogen peroxide is itself decomposed and consumed, thus causing a problem that the reaction is finished with the oxidization for producing copper oxide uncompleted.
In order to avoid a change of the treated waste liquid into the pasty object due to the precipitation of the solid material whose main component is the double salt, it is effective to dilute the waste liquid so that a concentration of copper ions is not more than about 10 g/L and a concentration of chloride ions or sulfate ions is not more than about 20 g/L during a neutralization treatment. However, in this treatment, there is a problem that a large amount of dilution water is required and accordingly a facility for this treatment becomes large.
Further, when treating a strong acidic waste liquid having copper ions and chloride ions or sulfate ions which coexist in high concentrations, such as an etching waste liquid containing copper ions, even if hydrogen peroxide is added to the acidic waste liquid containing copper in advance, a precipitate whose main component is the above-mentioned double salts is partly produced in the course of a neutralization reaction caused by adding an alkaline agent to the acidic waste liquid to change the liquid property from acidic to neutral or alkaline. Accordingly, most of hydrogen peroxide is catalytically decomposed and consumed. Then, the reaction of oxidization treatment for producing copper oxide is finished with the oxidization uncompleted due to a lack of the amount of hydrogen peroxide. In compensation for this, it is possible to improve the condition of oxidization treatment by adding hydrogen peroxide excessively to the waste liquid in anticipation of the lack of hydrogen peroxide during the oxidizing reaction. However, a large amount of additive chemicals is required, and thus such treatment is inefficient. Even in this case, the double salts which are not oxidatively decomposed with hydrogen peroxide still remain in sludge. A concentration of double salts can be reduced by rinsing the sludge with water sufficiently to remove the double salts from the sludge. However, a large amount of rinsing water is required and another treatment is required because copper ions are contained in rinsing wastewater, and thus such treatment is inefficient.
Furthermore, in these technologies, the treatment is conducted under the condition where a solution property is alkaline, i.e. a pH is in the range of eight to 12 by adding the alkaline agent to the acidic solution. Thus, neutralization treatment is required again in order to adjust a solution property to approximately neutral property in subsequent processes such as a dewatering process of the recovered solid material and a discharging process of a separated liquid. In this case, it is hard to say that this method is efficient because chemicals for re-neutralization are needed.
As described above, there has been no technology to recover only copper efficiently from copper-containing acidic waste liquids containing highly-concentrated copper ions and highly-concentrated salts like chloride ions which hinder the recovery and recycling of copper, and thus these waste liquids have been generally collected and discarded without recycling by industrial waste disposers.